1. Field of the Invention
The present invention relates to a tandem piston arrangement incorporated in hydraulic fishing jars to increase the effective piston area thereof whereby hydraulic pressure is maintained at a minimum level in the jar permitting application of maximum pulling load to the jar to, in turn, obtain maximum jarring load capability for a given size jar.
2. Description of the Prior Art
Various types of fishing jars are employed for moving some stuck tool or tubular member or other object from a well bore, the stuck object being referred to as a "fish". Fishing jars are run at the lower end of a string of drill pipe or tubing which ordinarily is referred to as the fishing, running or working string and the fishing tool is engaged with the lower end thereof. The fishing tool may be a spear, or overshot or similar device, adapted to engage the fish so that the fish may be jarred loose by the jar and thereafter retrieved from the well bore.
Jars are employed for the purpose of applying hammer blows to aid in releasing the stuck fish while the fishing string is under tension. Jars of the hydraulic type, in general, are quite well known and comprise telescoping members. Upon expansion, a pressure chamber containing a quantity of hydraulic fluid resists elongation of the jar. However, when an upward strain or tension is applied to the running string, the hydraulic fluid is compressed and bleeds through a restricted flow passage, thus permitting a gradual telescoping of the tool until a large by-pass is opened and the induced pressure on the hydraulic fluid is instantaneously released. Since the fishing string is no longer resisted by the compressed fluid, the jar telescopes rapidly until such telescoping is stopped by engagement of a hammer and anvil that form part of the fishing tool which applies a jar to the fish.
In hydraulic fishing jars, one or more hydraulic cylinders are provided for effecting the jarring mechanism. As the drill pipe is picked up at the top of the well, the parts of the tools telescope such that fluid in the cylinder is compressed. Pressure build-up in the hydraulic cylinder or cylinders is directly proportional to the amount of pull applied to the tool. In jars having a piston element within the cylinder element, the build-up is inversely proportional to the area of the piston because of the inherent functioning of a piston within a pressurized chamber.
During the complete operation of the jar, two types of loads can be identified. While the drill pipe is being stretched before the tool is released to cause the hammer to interface with the anvil, a "jarring load" is applied to the tool. The force exerted on the anvil during the hammer-anvil interface is defined as the "impact load". After tripping of the tool, the jar mechanism can tolerate a much higher jarring load because there is no longer a pressure build-up within the hydraulic cylinder in the tool. However, during the stretching of the drill pipe and prior to the tripping of the jarring mechanism, the tool is limited to the jarring or pulling load because of pressure build-up within the hydraulic cylinder. If a jarring load is applied to the tool in excess of the design limits of the tool, the control mandrel may collapse or the housing may burst.
Because of the restricted inner and outer diameters of the jar mechanism, there is, by necessity, a limitation in space and area for incorporation of the piston mechanism. To increase the piston head area to obtain a maximum diameter thereof, a seal having a comparatively small diameter could be put around the control mandrel for the jar at the upper end thereof such that the outer diameter thereof is as small as possible, and a larger seal may be applied to the piston head such that the largest possible area is obtained between the piston seal and the mandrel seal. To obtain such a maximum effective piston area in prior art jarring tools, hammer and spline mechanisms would have to be removed from within the piston chamber. However, when these components are put outside of the piston chamber, they are, of necessity, placed exteriorly of the hydraulic and lubrication systems. Therefore, milling cuttings and other debris within the well may become easily entrapped into spline areas and may also cause deterioration of the hammer, anvil and other surfaces. Therefore, it would be desirable to provide a means for obtaining the maximum effective piston area while, at the same time, including all of the operational components of the jarring mechanism, including the hammer and spline mechanisms, into the hydraulic and lubrication system.
Plural hydraulic systems could be designed into the tool, the first system being under pressure to provide hydraulic activation of the tool. The second system would not be under pressure, but would be a balanced lubrication system enclosing the splines and the hammer. Use of such plural hydraulic systems presents numerous problems. For example, one such design would require the filling of two separate chambers with fluid. Additionally, each of the chambers would have to be pressure compensated, such that pressure inside the tool is balanced with the hydrostatic pressure outside of the tool in the well at the depth of the operation of the tool so that the tool will not burst or collapse from temperature expansion and contraction, and the like.
The present invention overcomes these obstacles by providing one utilized hydraulic and lubrication chamber mechanism having a plurality of interrelated chamber members. A plurality of piston elements are provided within the chamber, in tandem series, and communicating with respect to one another, such that the area of each piston head provides a total effective piston head area whereby pressure within the tool may be maintained at minimum levels. Additionally, use of such an arrangement permits all working components to be in one lubrication system. Accordingly, the jarring mechanism can be operated at minimum pressures within the interior of the tool, and can tolerate the application of maximum pulling load within the design limits of the tool whereby maximum jarring load capability is afforded for a given size tool. Thus, a comparatively high load rating may be obtained for the tool.